This module implements string buffers that automatically expand
as necessary. It provides accumulative concatenation of strings
in quasi-linear time (instead of quadratic time when strings are
concatenated pairwise).

create n returns a fresh buffer, initially empty.
The n parameter is the initial size of the internal string
that holds the buffer contents. That string is automatically
reallocated when more than n characters are stored in the buffer,
but shrinks back to n characters when reset is called.
For best performance, n should be of the same order of magnitude
as the number of characters that are expected to be stored in
the buffer (for instance, 80 for a buffer that holds one output
line). Nothing bad will happen if the buffer grows beyond that
limit, however. In doubt, take n = 16 for instance.
If n is not between 1 and Sys.max_string_length, it will
be clipped to that interval.

Empty the buffer and deallocate the internal string holding the
buffer contents, replacing it with the initial internal string
of length n that was allocated by Buffer.createn.
For long-lived buffers that may have grown a lot, reset allows
faster reclamation of the space used by the buffer.

add_substitute b f s appends the string pattern s at the end
of the buffer b with substitution.
The substitution process looks for variables into
the pattern and substitutes each variable name by its value, as
obtained by applying the mapping f to the variable name. Inside the
string pattern, a variable name immediately follows a non-escaped
$ character and is one of the following:

a non empty sequence of alphanumeric or _ characters,

an arbitrary sequence of characters enclosed by a pair of
matching parentheses or curly brackets.
An escaped $ character is a $ that immediately folows a backslash
character; it then stands for a plain $.
Raise Not_found if the closing character of a parenthesized variable
cannot be found.